1) Field of the Invention
The present invention relates to an image forming apparatus that includes a fixing-unit system with a capacitor that supplies electric power to a heating section.
2) Description of the Related Art
In copy machines, printers, and facsimiles nowadays, it is a common technique to fix a toner image on a recording medium by heating toner in the toner image with a fixing unit. The toner image is formed by an electrophotography on the recording medium such as a transfer paper, and then, the recording medium is passed through the fixing unit.
In such a fixing unit, a roller or an endless belt is heated by heat from a heating section that is supplied with electric power. A fixing member such as the roller or the endless belt is allowed to make a contact with the recording medium to heat the toner. In this case, so far, the power has normally been supplied to the heating section by a commercial ac power supply. However, a fixing-unit system that supplies the power to the heating section together with a capacitor has been developed recently. An example of such a fixing-unit system is disclosed in Japanese Patent Application Laid-open Publication No. 2002-174988.
In an image forming apparatus, shortening of a total copying time from a start up of the apparatus up to an end of copying, has been sought after. While starting up by putting a main power supply ON when the fixing-unit system is at halt, if a power stored in the capacitor is not less than a threshold for starting electric discharge, the power can be supplied to a plurality of heating sections by respective commercial ac power supply and capacitor and a temperature can be raised up rapidly up to a reload temperature, thereby shortening a waiting time till the apparatus can be used, which in turn enables to shorten the total time required for copying.
However, even in such an image forming apparatus, the apparatus is structured to perform control by giving priority to securing an image quality. Therefore, while passing the paper continuously, if energy of power stored in the capacitor is less than the threshold for starting electric discharge, a CPM (copies per minute) down control in which number of images formed per minute or number of images fixed per minute (CPM) are reduced, i.e. paper-passing speed is reduced to a speed slower than a normal speed, has been performed for preventing defective fixing.
Apart from this, while passing a cardboard paper or passing a paper with a large image data to be transferred, due to a very big drop in a temperature of the fixing member, in a case of the cardboard paper for example, the CPM down control called as a board-paper mode is performed to prevent the defective fixing and to secure the image quality.
Therefore, an image forming apparatus that includes a capacitor, has been provided with a control mode for normal CPM conditions, the CPM down control mode for a shortage of power stored in the capacitor, and a CPM down control mode for passing the cardboard paper. In order that each mode is use appropriately, the structure of the control system has been complicated.
Moreover, while passing the cardboard paper, a drop in temperature of the fixing member is detected while the cardboard paper is being passed and for reducing the CPM, a time is required to adjust the CPM in the image forming apparatus, thereby resulting in more time required for copying.